FIG. 1 illustrates an MRAM cell 10P with MTJ 200P in which the easy axes of the magnetic layers are perpendicular to the plane of the thin films as illustrated by the arrows on the magnetic layers 15, 13, 11. MTJs can also be designed with in-plane magnetization. An MRAM cell structure typically includes a top metal contact and a bottom metal contact. The metal contacts are also referred to as electrodes.
A typical STT-MRAM (spin transfer torque magnetic random access memory) MTJ (magnetic tunnel junction) stack has a reference layer 13 whose magnetization is fixed in a certain direction by either intrinsic anisotropy field, or through an exchange coupling field from an adjacent pinned magnetic layer. In the example shown in FIG. 1 reference layer 13 is antiferromatically coupled through AF-coupling layer 14 to the pinned layer 15. The MTJ also has a switchable free magnetic layer 11, whose magnetization direction can be switched relative to that of the reference layer 13 by an electric current flowing between the reference layer and free layer through a junction layer 12, typically an oxide of Mg, Al and Ti, or a metallic layer of Cu, Au, or Ag. The different relative angles between free layer and reference layer magnetization directions gives different resistance levels across the MTJ stack. The free magnetic layer has a magnetization direction that is switchable in either of two directions. The resistivity of the whole MTJ layer stack changes when the magnetization of the free layer changes direction relative to that of the reference layer, exhibiting a low resistance state when the magnetization orientation of the two ferromagnetic layers is substantially parallel and a high resistance when they are anti-parallel. Therefore, the cells have two stable states that allow the cells to serve as non-volatile memory elements.
Reading the state of the cell is achieved by detecting whether the electrical resistance of the cell is in the high or low state. Writing the cells requires a sufficiently high DC current flowing in the direction through the MTJ stack between the top and bottom metal contacts to induce a spin transfer torque (STT) that orients (switches) the free layer into the desired direction. The amount of current needed to write the cells is at least slightly higher than the current that flows during the read process, so that a read operation does not change the state of the cell.
A recent study by Wang, et al. on perpendicular MTJ shows that the perpendicular anisotropy of magnetic layers in MgO based MTJ structures can be changed by the voltage applied to the magnetic layers. See Wei-Gang Wang, et al., “Electric-field-assisted switching in magnetic tunnel junctions”, Nature Materials Vol. 11, 64-68 (2012).
Wang, et al. used an example MTJ layer structure (which is illustrated in FIG. 2 herein) of bottom magnetic layer 43 CoFeB (1.3 nm), MgO layer 42 (1.4 nm), and top magnetic layer 41 CoFeB (1.6 nm). In the test setup as shown a small positive DC electric potential is applied to the MTJ cell to drive electrons into the bottom magnetic layer. When MgO layer 42 is thick enough and resistance across the MgO junction is high enough, the current density through the MgO junction will be low. In this case, the two magnetic layers adjacent to the MgO layer form a capacitor across the MgO layer, which is fundamentally the same as a classic parallel-plate capacitor with the MgO layer as the dielectric between the parallel plates. When a voltage is applied to the MgO junction, electrical charges will accumulate in the two magnetic layers, which is governed by the capacitor equation of Q=C×V, where Q is the net charge, C the capacitance and V the applied voltage. The applied positive voltage as shown in FIG. 2 causes the top magnetic layer to have a positive potential, i.e. electron depletion at the top layer's interface to the MgO layer. Wang's graph (reproduced in FIG. 3 herein) shows the coercivity field Hc for the top and bottom layers as a function of the electric field. The perpendicular anisotropy is reflected by the measured coercivity field Hc. With increasing applied voltage and electron depletion at the top layer's interface to the MgO layer, the top magnetic layer shows increased perpendicular anisotropy. The layer that has negative potential (the bottom layer in this example) and, therefore, electron concentration at the layer's interface to MgO, shows decreased perpendicular anisotropy.
When voltage is applied to an MgO junction having effective capacitance, the magnetic layer having a negative voltage potential will have net negative charges, which are basically conductive electrons, accumulated at its interface with the MgO layer. For the magnetic layer having positive potential, the positive charges at its interface with MgO are basically vacancies of conductive electrons that are depleted by the applied voltage. Unlike a standard capacitor, in an MgO/magnetic junction acting as a capacitor the electrons at the magnetic layers' interfaces also affect the magnetic anisotropy in those layers. The Wang article cited above as well as the Ikeda, et. al. article show that surface perpendicular anisotropy of CoFeB layer on the junction with the MgO layer is intrinsically due to the broken-symmetry of the interface CoFe lattice of the CoFeB layer facing the MgO layer. See S. Ikeda, et. al., “A perpendicular-anisotropy CoFeB—MgO magnetic tunnel junction”, Nature Materials Vol. 9, 721-724 (2010).
In a perfectly symmetric and continuous lattice of CoFe, the electron-to-electron spin exchange coupling between the unpaired 3d-electrons, which are also the conductive electrons, of Co and Fe atoms cancel out each other's effect and produce zero anisotropy energy in a symmetric and continuous lattice. However, at the interface of the CoFeB layer and MgO layer, the CoFeB layer's interface is actually CoFe facing MgO. MgO breaks the symmetry of the CoFe lattice, so that the 3d-electrons of Co and Fe atoms at the interface lose their cancellation-counter-part and create a net anisotropy energy that produces an effective anisotropy field perpendicular to the interface plane. Thus, the originally soft magnetic CoFeB film can exhibit strong perpendicular anisotropy on the sides of the MgO and show hard magnetic behavior.
Wang, et al. show that with applied voltage, the magnetic layer that has a positive potential, i.e. electron depletion at the layer's interface to MgO, shows increased perpendicular anisotropy. The magnetic layer that has negative potential, i.e. electron concentration at the layer's interface to MgO, shows decreased perpendicular anisotropy. A possible cause of such behavior can be that the magnetic layer having increased electrons will have more conductive electrons filling in the 3d-band of the interface CoFe lattice and reducing the unpaired 3d-electron population. This makes the broken-symmetry induced surface perpendicular anisotropy weaker, and thus makes the magnetic layer magnetically softer. When 3d-electrons are depleted in a magnetic layer, electrons will be depleted first from paired 3d-electrons due to Hund's Rules. More 3d-electrons becoming unpaired enhances the surface perpendicular anisotropy and makes the layer magnetically harder to switch by external field or spin transfer torque (STT).
Capacitance of MgO based magnetic tunnel junctions (MTJs) will be discussed next. For an MTJ with resistance-area (RA) product of 100 Ωμm2 or less, which is a common value for MTJ used in conventional MRAM cells, the barrier layer is thin enough to allow a higher density of electron current to tunnel through the barrier and results in effective resistance of a patterned MTJ stack within a few hundreds to a few thousand ohms. However, due to this low RA and high current density, when a voltage is applied to the MTJ, charges (accumulated and depleted electrons) will not be held within the magnetic layers adjacent to the barrier layer to provide a capacitive effect. The MTJ acts more like a conductor as the charges leak to the other side of the barrier layer due to the voltage potential. Therefore, the capacitance of low RA MTJ patterned stacks is generally very low. Prior study by Aoki1, et. al. shows that for an RA=5.5 Ωμm2 MTJ stack with an MgO barrier, the capacitance of a 100 nm×200 nm size patterned MTJ cell, including contribution from the larger top and bottom leads, is only 14 femtofarads (fF). For the capacitance from the magnetic layers and MgO barrier, it is only a few fF. See T. Aoki1, et. al., “Fabrication of MgO-based magnetic tunnel junctions for subnanosecond spin transfer switching”, Journal of Physics: Conference Series 266, 012086 (2011).
However, for MTJ with RA of 10 k Ωμm2 or higher, at MgO thickness of 2 nm or thicker, the tunneling current density is much lower so that charges can accumulate in the magnetic layers adjacent to the MgO barrier and thereby form an effective capacitor. Prior study by Padhan, et. al. shows that MTJ effective capacitance has an inverse linear trend versus MgO thickness. See P. Padhan, et. al., “Frequency-dependent magnetoresistance and magnetocapacitance”, Appl. Phys. Lett. 90, 142105 (2007) and FIG. 4 herein. The inset in FIG. 4 shows Padhan's RC circuit equivalent of the MTJ with a series combination of an interface capacitance Ci, and a bulk capacitance C from the MgO. For example, at MgO layer thickness (tMgO)=2.5 nm, with RA˜100 k Ωμm2, area/C is about 0.005 cm2/μF. From Wrona, et al., for MTJ with RA˜10 Ωμm2, critical dimension (CD)=65 nm, effective capacitance is only ˜1 fF, with tMgO˜1.0 nm.
Also from Padhan, et. al., A/C versus tMgO trend indicates that at certain thickness between 2.0 nm˜2.5 nm, NC can be very close to 0 but still be a positive value, meaning effective C can be relatively high. From the trend line of Padhan, et. al., A/C˜0.0006 at tMgO˜2.44 nm, where RA˜100 k Ωμm2. At CD=65 nm, effective capacitance is ˜0.13 pF.
For STT-MRAM to provide distinctive resistance state changes, the simple design with a fixed reference layer and switchable free layer suffices for the purpose. However, if reference layer magnetization could also be switched under certain conditions, while staying un-perturbed during conditions that switch the free layer, then the STT-MRAM could be used as a more powerful logic element providing sophisticated logic functions than a simple data storage memory based on resistance-level.